Create Fuel role with nailgun api/cli

Currently role is fetched from Release.roles_metadata which is quite bad limitation, and doesnt allow
to freely hack on Fuel.

This blueprint proposes to make several internal changes in db structure and provide api/cli interface for creating role with API.

External API should take a form of:

POST/PUT release/<id>/role

{ name: 'redis',
  description: 'Redis server',
  volumes: {'id': 'os', 'allocate_size': 'min'}
  other_ui_meta...}

GET /release/<id>/role/<name>

DELETE /release/<id>/role/<name>

From cli it should be like:

fuel rel --rel 1 role create --data release_1/redis.yaml

Role DB object should be extended to include all fields added to API.

UI should fetch roles info not from Release.roles_metadata but from separate API call